Parent Company Financial Information, Condensed Income Statements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Income Statements [Abstract]
Securities gains	$ (644)	$ 3,087	$ 92
Interest, dividend and other income	286,947	273,224	275,081
Income before income tax benefit and equity in undistributed income of subsidiaries	118,801	116,628	112,303
Income tax benefit (expense)	(40,392)	(40,203)	(37,229)
Net income	78,409	76,425	75,074
NBT Bancorp Inc [Member]
Condensed Income Statements [Abstract]
Dividends from subsidiaries	10,200	78,200	35,400
Management fee from subsidiaries	95,244	92,629	87,116
Securities gains	652	3,034	49
Interest, dividend and other income	976	693	800
Total revenue	107,072	174,556	123,365
Operating expense	97,977	94,332	89,834
Income before income tax benefit and equity in undistributed income of subsidiaries	9,095	80,224	33,531
Income tax benefit (expense)	321	(515)	631
Dividends in excess of income (equity in undistributed income) of subsidiaries	68,993	(3,284)	40,912
Net income	$ 78,409	$ 76,425	$ 75,074